Orange scions on sour orange appear to be most susceptible to tristeza, followed by mandarin scions. When tristeza infected the old Citrus Variety Collection at Riverside, which was on sour orange, most of the orange cultivars were in advanced stages of decline before the first mandarin combinations began to express symptoms.Young grapefruit trees on sour orange are fairly susceptible to tristeza and decline readily when inoculated, but generally do not collapse. With mature grapefruit trees under field conditions in California, the decline is very slow. None of the taxa within the sour orange group have shown any indication of stem pitting from the presence of tristeza. However, grapefruit trees on sour orange, in the absence of tristeza, do have a tendency for inverse pitting to occur in the sour orange immediately below the bud union. Perhaps the best known of these is the ‘Natsudaidai’ or ‘Summer daidai’ of Japan. It is C. natsudaidai and is grown commercially as a grapefruit substitute. In the tristeza trials at South Coast Field Station in California, Bitters found it to be tolerant to tristeza with Valencia orange scions. The trees were vigorous, but the trees were shy bearers, a characteristic known in Japan where it is occasionally used as a root. The author never included it in any horticultural evaluation trials and it has never been used commercially except for the few trees in Japan. The ‘kikudaidai’, or ‘chrysanthemum daidai’,frambueso en maceta because of the natural ridging of the fruit, is C. canaliculata. In the South Coast Field Station experiments , it was susceptible to tristeza.

For further evaluation, it was placed in a lemon root stock evaluation trial by the author at Lindcove Field Station, Tulare County, in 1977. It has shown no merit as a root stock there for lemons, oranges or tangelos. The most tested of these probable hybrids is the ‘Nansho daidai’ of Japan. It is classified as C. taiwanica. Salibe states that it is tolerant to tristeza in Brazil. This cultivar was included in the root stocks tested for tristeza at Baldwin Park, California . The combinations were rated susceptible since the inoculated trees are quite stunted, but somewhat questionably so. A picture of these trees at 13 years of age is shown in Figure 80. The inoculated tree is on the right, uninoculated on the left. This combination illustrates an average response, not the best, but the poorest . In Florida, Garnsey reported that the nansho daidai was susceptible to the T3 strain of tristeza. This is the author’s proof. Where is the evidence of those who say it is resistant? Because of its somewhat questionable tolerance to tristeza, it was incorporated into a number of root stock evaluation trials in California, Arizona, Florida, Texas and Brazil. In Brazil, Moreira et al. report that yields and fruit quality of both grapefruit and Valencia orange were poor. In Texas, Wutscher and Shull ; Wutscher, Maxwell, and Shull ; and Wutscher and Dube report that the yield of early orange varieties and tangelo on C. taiwanica was similar to trees on sour orange, but that the fruit quality was also low . In California, the author planted root stock trials containing the stock with lemons, navels, Valencia oranges, and Marsh grapefruit in Tulare, Ventura, Riverside and Orange counties. Like the Brazil experience, yields and fruit quality were poor. In fact, in the South Coast Field Station plantings in Orange County, California, the granulationof the fruit on C. taiwanica was so severe the fruit was harvested in advance of the normal harvest period and actually dumped. None of the other 24 root stocks in the trial had this problem. These trees are now 28 years old, and still show no general decline from tristeza. The only good experimental result on this stock was in a root stock planting at Yuma, Arizona, on a very sandy soil.

In this planting, Rodney and Harris found the highest yields on this stock. Trees on C. taiwanica are quite cold hardy. A personal communication from Harold Ormsby of the Yuma, Arizona, area indicated they were the most cold tolerant of any trees he’d grown in the Yuma area. This cultivar has nothing to recommend it for root stock use. It is highly variable from seed. The fact that it produces so many hybrids has made it attractive to some plant breeders, but it should not be. The ‘Karna Khatta’ is probably another sour orange hybrid and is classed as C. karna. In the Baldwin Park experiments, Bitters and Parker found it tolerant to tristeza. The trees were vigorous, but somewhat shy bearers and quite susceptible to gummosis. These characteristics have been noted in India where it has occasionally been used as a root stock. Wutscher suggests that the ‘yama-mikan’ or ‘mountain mandarin’ classified by Tanaka as C. intermedia, may also be a sour orange hybrid. This might be a questionable issue. The question has nothing to do with the results reported. The question deals with Wutscher’s statement that the ‘yama-mikan’ is a sour orange hybrid. The name suggests that one parent of this hybrid might be mandarin, the other parent is debatable. However, Tanaka places this cultivar with pummelo hybrids and perhaps with no evidence to the contrary, that is what it should be considered as. It is occasionally used as a root stock in Japan , and the author did see a few trees budded on it in his many trips to Japan. Wutscher, Maxwell, and Dube reported that mandarin hybrids budded on sour orange performed better than on yama-mikan. In the South Coast Field Station tristeza experiments, Bitters found Valencia oranges on this stock were susceptible to tristeza. It was never placed in any other root stock trials in California. The smooth flat Seville or Australian sour orange is, according to Hodgson , probably a hybrid. It has not been used as a root stock in Australia, according to Bowman , but according to Grimm and Garnsey , there has been some interest in Smooth Seville in Florida because it has some tolerance to gummosis as well as some tristeza tolerance. In Texas, Wutscher, Maxwell, and Shull found that yield and fruit quality of grapefruit on this root stock was poor. The author doesn’t know for sure whether the smooth Flat Seville is the same as the Mildura sour of Australia.

The latter is not a typical sour, and while it was reported to have some tristeza resistance, this fact has been disproven. Another cultivar which is a possible sour orange hybrid is C. obovoidea, sometimes known as the ‘marumero’ orange, but best known in Japan as ‘kinkoji’. In Japan, it has been used as an indicator plant for seedling yellow tristeza, but in the author’s tristeza trials at South Coast Field Station , when inoculated with normal strains of tristeza, it shows good tolerance. Along with other tolerant or lesstolerant stocks that did well at South Coast Field Station, the author transferred seed to the Lindcove Field Station in Tulare County to place in long term rotation trials. Rootstock plantings of Limoneira 8A Lisbon containing trees on C. obovoidea, C. miaray, C. canaliculata, C. neo-aurantium, C. shunkokan,planta de arandanos en maceta and other stocks, and Parent Navel, Olinda Valencia, and Minneola tangelo scions on C. miaray, C. obovoidea, C. neoaurantium and C. shunkokan, as well as other stocks, were planted out in replicated plots by the author in 1977. Upon the author’s retirement in 1982, the root stock work wasassigned to Dr. M. L. Roose of the Department of Botany and Plant Sciences, University of California, Riverside. Roose, in 1985, published a preliminary progress report on these root stock plantings , and a summary report in 1989 . This is ten years from the planting date and provides about seven years of yield data. In his report, Roose eliminates the possible further testing of C. miaray, C. canaliculata, C. neo-aurantium, C. shunkokan, and C. obovoidea and some others as having no value as root stocks. Perhaps he’s right in most instances, but each of the root stocks placed in those trials was included because of a purpose—they were not pulled out of a hat. While the grower is interested in such a progress report, the grower also wants to know what those trees will be doing at 20 years, 30 years, and even 50 years. A ten-year decision with no other experience is not adequate. Let me cite several points in partial defense of C. obovoidea. Some extra trees of the various root stocks contained in the Lindcove plantings were distributed from that nursery by C. D. McCarty to some cooperating growers in Tulare, Ventura and Riverside counties and planted in 1977. In August, 1988, the author had a communication from R. M. Burns of Ventura , in which he states that with Valencia orange planted in 1977 , that the rating of the trees on the six root stocks included found that the trees on C. obovoidea were the best. In 1987, I received a letter from a grower-friend of the Amanzi Estates near Uitenhage in the Sunday’s River area of South Africa.

In this communication, Mr. Niven asks the question “What can you tell me about C. obovoidea? It looks rather interesting.” A good long hard look should be taken at C. obovoidea. If it doesn’t have the qualities to make it as a root stock per se, then perhaps it would be worth hybridizing. Among other possible sour orange hybrids looked at by the author was the ‘konejime’, or Citrus neo-aurantium. This cultivar is also called ‘Tosu’ and ‘Takosu’. It was tolerant to tristeza in the author’s 1969 plantings at South Coast Field Station . It should also be looked at in depth. The ‘Rokugatsu’ or ‘Tanakas’, C. rokugatsu, was very susceptible to tristeza . The ‘otachibana’, Tanaka’s C. otachibana, was also very susceptible to tristeza . C. miaray was also susceptible to tristeza , but was placed in a lemon root stock trial by the author at Lindcove Field Station in 1972. Its performance there has been poor . C. maderspatana or ‘vadlapudi’, sometimes ‘kitchili’, was susceptible to tristeza . In 1971 plantings at South Coast Field Station , the ‘Natsudaidai’, or C. natsudaidai, was tolerant to tristeza. The trees were vigorous, but shy bearers, a fact known in Japan. The author never placed them in any other root stock trials. In the Baldwin Park plantings , the ‘karna khatta’, or C. kharna, was also tolerant to tristeza. The trees were vigorous, but very susceptible to gummosis, a trait known in India and Pakistan. The author never tested it further. The ‘gabbuchinee’ has been tested in Pakistan and India, but has been of no commercial importance. The ‘leather-head’ sour orange described by Swingle is probably as he indicates, a sour orange hybrid. Reports from China have indicated this cultivar is tolerant to tristeza. In China, this variety is called Pi-do-geng. C. A. Roistacher, Plant Pathologist at the Citrus Research Center, Riverside, visited China in 1981 and l986. He found mature trees of this variety heavily stem-pitted by tristeza. A planting made by Roistacher at the Citrus Research Center comparing Valencia trees on the leather-head sour with Valencias on a standard sour orange, and inoculated with several sources of tristeza, show quite a contrast . There were different reactions to the different sources of tristeza inoculum. The sour orange, of course, was very susceptible to all sources of the virus. Trees on the leather-head sour were very compact and dark green three years after the inoculation, although the sour orange counterparts were in severe decline. Unfortunately, Roistacher left no checks. To the author, the trees on the leather-head sour appear stunted and lacking in general vigor. In other words, there is a slight tristeza reaction, which may differ with different sources of the tristeza virus and perhaps the environment. This hybrid also should be hybridized. Some of the ‘citradias’, hybrids between sour orange and trifoliate orange, were tested at Baldwin Park, California , but the results were extremely variable. Additional hybrids of this combination are needed. In 1980, C. D. McCarty, Horticultural Technologist at the Agricultural Cooperative Extension, Citrus Research Center, made numerous crosses between sweet orange and sour orange in the Citrus Variety Collection at Riverside. His objective was to obtain hybrids which might be tolerant to both tristeza and gummosis.